Gumbelevicius U.S. Pat. No. 3,788,914 discloses a composition and process of chemically milling titanium, refractory metals, and their alloys with little hydrogen absorption in the alloys and uniform titanium removal rates. The solution comprises nitric acid, hydrofluoric acid, and derivatives of carbonic acid. The solution may also include surfactants when proper fillet radii configuration of the chemically milled parts is important. Suitable surfactants include dodecyl benzene sulfonic acid and alkyl benzene sulfonate.
Coggins et al U.S. Pat. No. 3,944,496 discloses a solution for chemical milling which contains nitric acid, hydrofluoric acid, phosphoric acid, and carbamide. This composition too, can contain a surfactant.
Gumbelevicius Ser. No. 659,770 filed Feb. 20, 1976, entitled CHEMICAL MILLING OF TITANIUM AND REFRACTORY METALS discloses a composition for pickling and chem-milling in which the nitric acid is eliminated but which includes a surfactant.
The reduction of surface tension (the function of surfactants) is required for extremely accurate work. However, the use of the prior art surfactants results in several problems in chemical milling in the field.
The use of surfactants leads to difficulty in maintaining the surface tension of the solution at a constant level due to decomposition of the surfactant itself. The varying level of surface tension results in a variance in metal removal rate, and "grooving" along chem-mill lines.
The use of surfactants also causes foaming. Foaming is undesirable in that it limites the number of parts which can be chemically milled at one time.
Furthermore, the alloying elements in the titanium alloys and the refractory metal alloys (primarily aluminum) slowly form hard scales which adhere tightly to surfaces of the equipment in which the alloys are being milled, and particularly to the heat exchangers. This results in difficult equipment maintenance problems. The addition of sodium nitrate causes the formation of loose, non-adherent, easily removable sludge, rather than hard, adherent scale.
It has been discovered that problems associated with surfactants can be eliminated by replacing the surfactant with sodium benzoate. In addition, sodium benzoate improves the flow characteristics of the chem-milling solution. This improvement of flow characteristics eliminates excessive metal removal in the fillets. This metal removal, known as "grooving", is a normal occurrence when the surfactant is not present or when it has been depleted to a very low level. Unlike conventional surfactants, large amounts of sodium benzoate do not cause excessive metal removal in the center of the chem-milled area. This type of excessive removal is known as "dishing". Furthermore, sodium benzoate eliminates tapering along the fillet lines which is detrimental for high precision chem-milling.